It has long been known that both heavy metal compounds and amines are efficient catalysts for isocyanate polyaddition reactions (see K. C. Frisch, L. P. Rennao, "Catalysis in isocyanate reactions" in J. Macromol. Sci.-Revs. Macromol. Chem. C5 (1), 103-150 (1970)).
It is also known that metal compounds and amines may have a synergistic effect on each other for reactions of polyisocyanates with polyols (see J. H. Saunders and K. C. Frisch, Polyurethanes, Part I, Interscience Publishers, New York -- London -- Sydney, 1962, pages 231, 232).
The most catalytically active amines, which generally contain tertiary nitrogen atoms (such as 1,4-diazabicyclo-(2,2,2)-octane) are generally required to be used in amounts of from 0.04 to 0.5 parts by weight, based on the total quantity of polyol in the reaction. Considerably larger catalyst quantities are required if other amines are used as catalysts or if the amines are to catalyze reactions of aliphatic isocyanates.
However, even the use of small quantities of amine catalysts entails serious disadvantages. Since the catalysts used for industrial processes are generally tertiary amines which are not chemically built into the polyurethane, a troublesome odor of amine persists long after production of the polyurethane resin. This is particularly unpleasant in articles for daily use, such as cushions or the like, equipment for car interiors, shoes, furniture, and the like. Moreover, amines frequently cause yellowing of light colored leather or plastic surfaces.
Bicyclic amidines have been described as catalysts for polyurethanes in U.S. Pat. No. 3,814,707 and in British Patent No. 1,182,014. These publications also mention the addition of organometallic compounds.
Free amidines have been described as catalysts for reactions of aliphatic isocyanates in German Offenlegungschrift No. 1,950,262, but these compounds have the disadvantage of being readily decomposed by water (see Houben-Weyl-Muller, Methoden der organischen Chemie, published by G. Thieme, Stuttgart, Volume XI, page 940). This substantially excludes the use of foam systems in which water is used as a blowing agent. Even the traces of water invariably present in polyols are sufficient to reduce the catalytic activity of free amidines within a few days. A polyol mixture containing an amidine catalyst may therefore only be stored for a limited length of time.
In addition to water, numerous halogenated hydrocarbons, such as CH.sub.2 Cl.sub.2, CHCl.sub.3, CCl.sub.4, HClC.dbd.CClH, HClC.dbd.CCl.sub.2, H.sub.2 ClC-CClH.sub.2, and the like severely restrict the possible uses of amidines because they react very rapidly to quaternize amidines. This renders them virtually useless as catalysts.